The present invention relates to a color cathode ray tube, and particularly, to a color cathode ray tube which displays a high quality image by reducing elliptic deformation of beam spot shape in the peripheral portion of a fluorescent screen.
In general, an inline color cathode ray tube comprises an inline electron gun which emits a three electron beams including a center beam and a pair of side beams which run on one same plane and are arranged in one line in the horizontal direction. The three electron beams emitted from the inline electron gun are concentrated by themselves on a fluorescent screen by a non-uniform magnetic field which generates a deflection yoke, i.e., a pin-cushion type deflection magnetic field formed in the horizontal direction, and a barrel type deflection magnetic field formed in the vertical direction.
Various methods are known as an inline type electron gun as described above, and an electron gun adopting a Dynamic Astigmatism Correct and Focus method is one of those method. An electron gun adopting the Dynamic Astigmatism Correct and Focus method comprises three cathodes K arranged in one line in the horizontal direction, and first to fourth grids G1 to G4 arranged in this order in a direction from the cathodes K to a fluorescent screen. The third grid G3 includes two segments G3-1 and G3-2. Each of the grids G1 to G4 has three electron beam pass holes arranged in one line in the horizontal direction, in correspondence with the three cathodes K also arranged in one line in the horizontal direction.
In this electron gun, each of the cathodes K is applied with a voltage of about 150 V and the first grid G1 is grounded. The second grid G2 is applied with a voltage of about 700 V. Each of the first and second segments G3-1 and G3-2 of the third grid is applied with a voltage of about 6 kV. The fourth grid G4 is applied with a high voltage of about 26 kV.
By applying these voltages, the cathodes K, the first grid G1, and the second grid G2 constitute an electron beam generating section, and a virtual object point is formed with respect to a main lens described later. The second grid G2 and the first segment G3-1 constitute a pre-focus lens for preliminarily focusing electron beams emitted from the electron beam generating section. The second segment G3-2 and the fourth grid G4 constitute a main lens for finally focusing the electron beams preliminarily focused, onto a fluorescent screen.
In this electron gun, when electron beams are not deflected but run forwards to the center of the fluorescent screen, voltages of an equal level are applied to the first and second segments, and electron beams emitted from the electron beam generating section are focused onto the center of the fluorescent screen by the pre-focus lens and the main lens.
In case where electron beams are deflected to the periphery of the fluorescent screen, a predetermined voltage is applied to the second segment G3-2 in correspondence with a deflection amount of the electron beams. This voltage changes so as to gradually increase parabolically such that the voltage is lowest when electron beams are focused to the center of the fluorescent screen and the voltage is highest when electron beams are deflected to a corner of the fluorescent screen. When electron beams are deflected to a corner of the fluorescent screen, the potential difference between the second segment G3-2 and the fourth grid G4 is smallest and the intensity of the main lens is weakest. Simultaneously, a quadrupole lens is formed by a potential difference between the first segment G3-1 and the second segment G3-2 and the intensity of this lens is strongest. This quadrupole lens is arranged so as to cause convergence in the horizontal direction and divergence in the vertical direction. The quadrupole lens functions to correct a focus displacement caused by an increase of the distance which electron beams run before arriving at the fluorescent screen, and to also correct a deflection aberration generated by a pin-cushion type horizontal deflection magnetic field and a barrel type vertical deflection magnetic field of defection yokes.
However, as shown in FIG. 2A, a deflection aberration cannot be sufficiently corrected by an inline color cathode ray tube comprising a normal inline electron gun. Therefore, there is a problem that a beam spot B1 of an electron beam which has arrived at a center portion of the fluorescent screen has a substantially circular shape while a beam spot B2 of an electron beam deflected to a peripheral portion of the fluorescent screen is deformed to be longer in the horizontal direction. Specifically, the beam spot B2 is formed such that a core portion 1 of high luminance expanded in the horizontal direction and having a elliptic shape is surrounded by a halo portion 2 of low luminance expanded in the vertical direction.
In response to the above problem, according to an electron gun adopting a Dynamic Astigmatism Correct and Focus method, a halo portion 2 of the beam spot B2 deflected to a peripheral portion of a fluorescent screen is eliminated as shown in FIG. 2B by correcting a deflection aberration as described above, so that electron beams are subjected to focusing over the entire fluorescent screen. However, in this kind of electron gun, elliptic deformation remains and the beam spot B2 is deformed to be longer in the lateral direction, at end portions of the horizontal axis H and the diagonal axis of the fluorescent screen. Therefore, moire is caused by an interference with electron beam pass holes in a shadow mask, so that the image quality of an image constituted by beam spots are degraded.
As a countermeasure for the above-described drawback, as shown in FIG. 1, grooves elongated in the lateral direction are formed in the side of the second grid G2 opposed to the first segment G3-1, thereby to weaken the focusing effect in the horizontal direction H caused by a pre-focus lens constructed by second grid G2 and the first segment G3-1 and to strengthen the focusing effect in the vertical direction V also caused by the pre-focus lens. Consequently, the diameter of the virtual object point in the horizontal direction H is reduced in relation to the main lens, and the diameter thereof in the vertical direction V is enhanced. As a result of this, the vertical diameter of a beam spot of an electron beam which has arrived at the fluorescent screen is enlarged, so that elliptic deformation of beam spots in the peripheral portion of the fluorescent screen is absorbed and moire is reduced.
However, in the method as described above, as the depth of a laterally elongated groove formed in the second grid G2 increases, elliptic deformation of a beam spot B2 is more absorbed in the peripheral portion of the fluorescent screen, while the vertical diameter of a beam spot B1 at the center portion of the fluorescent screen is enlarged so that the beam spot B1 is longitudinally elongated as shown in FIG. 2C. As a result, the resolution is degraded at the center portion of the fluorescent screen.
Specifically, where a priority is given to easy view of a displayed image at the center portion of the fluorescent screen, an image is degraded at the peripheral portion of the fluorescent screen. On the contrary, where a priority is given to a easy view of a displayed image at the peripheral portion of the fluorescent screen, the image is degraded at the center portion of the fluorescent screen. Thus, a conventional technique has a problem that a compromising design must be chosen for the entire fluorescent screen.
As described above, in order to obtain excellent image quality of a color cathode ray tube, excellent focusing characteristics relating to electron beams must be maintained over the entire fluorescent screen, and elliptic deformation of electron beam spots must be reduced. In a conventional electron gun adopting a Dynamic Astigmatism Correct and Focus method, by changing the intensity of the main lens in synchronization with a deflection current and by simultaneously forming the quadrupole lens, vertical halo portions of electron beams caused by a deflection aberration can be eliminated and focusing can be achieved over the entire fluorescent screen.
However, elliptic deformation of beam spots deformed to be laterally elongated at the peripheral portion of the fluorescent screen is apparent. If laterally elongated deep grooves are formed in the second grid G2 in order to absorb elliptic deformation of a beam spot at the peripheral portion of the fluorescent screen, the vertical diameter of a beam spot at the center portion of the fluorescent screen is enlarged and the resolution is degraded.